In order to test components for Coarse Wavelength Division Multiplexing (CWDM) and Fiber-to-the-Home (FTTH) passive optical networks, including the characterization of wide-band couplers, and the measurement of single-mode-fiber attenuation, there is a need for a wideband tunable laser that can be “continuously swept” over the bands of wavelengths used in such networks, typically encompassing 1250 nm to 1650 nm, and exhibit stable (especially repeatable) laser output over the entire wavelength range. Such a widely-tunable laser apparatus having a single gain medium is not commercially viable at this time, since not only is there not yet a readily available gain chip capable of covering this full spectral range, but it is also very difficult to build a tunable filter (necessary in a laser cavity to enforce oscillation at a desired wavelength) that has a low, approximately uniform insertion loss and spectral passband for any selected central wavelength falling within such a large (˜400 nm) range.
It is known, therefore, to use three or four conventional tunable external-cavity lasers, each tunable over a segment of the desired wavelength range (e.g. 1250 to 1650 nm) and select their respective outputs by means of a 1×N optical switch, switching from one to the next so as to cover the required wavelength range. An example of such a device is marketed by Santec Corporation as the Full-Band Tunable Laser TSL-210VF.
A disadvantage of such devices is that the output is not really continuous as the device is wavelength-swept throughout the entire range, because glitches or discontinuities occur at the transitions when the optical switch switches between the outputs of the different lasers.
There are two approaches for performing a swept wavelength measurement with this device. One is to stop the scan of a first one of the conventional external-cavity tunable lasers when its lasing wavelength attains a value where a second one of the conventional tunable external-cavity lasers is able to lase with at least as much power and signal-to-noise ratio. An optical switch is then used so that the light from the first tunable laser is blocked from being launched into the output fiber of the widely-tunable device and the light from the second tunable external-cavity laser is then launched into the same output fiber.
In order to avoid glitches or discontinuities in the wavelength when switching between respective outputs of two such lasers, their wavelengths must be synchronized to be substantially identical, within the limits of the desired wavelength resolution of the swept measurement, ultimately limited to approximately the linewidth of the laser emission. However, tunable external-cavity lasers typically exhibit a very narrow spectral output, which, even when artificially broadened using the “coherence control” feature offered with most commercial versions of these lasers, rarely can exceed an effective linewidth of more than 4 pm. The attainment of such a high degree of synchronization using two independent lasers can be difficult and prohibitively expensive. Moreover, this approach normally involves stopping the scan briefly during the measurement process, and leads to an abrupt change in the laser light intensity being output from the widely-tunable device as switching occurs. This prevents such a widely-tunable device from being used to perform rapid continuous scans across the full, say, 1250-1650 nm spectral range.
The second approach involves optically combining, via a coupler or splitter, the outputs from the constituent external-cavity lasers of the widely-tunable device. In this way, there can be, in principle, a smooth transition between the emission of one of the external-cavity lasers to the other as the widely-tunable device is scanned. However, the synchronization of the wavelength is even more difficult than in the “switched-output” case above, as the synchronization must be nearly perfect (i.e. to within the limits of the desired wavelength resolution of the swept measurement) not just at one wavelength in the region where the optical outputs of the adjacent tunable external-cavity lasers overlap, but at all wavelengths in a range of wavelengths (typically 5-20 nm) corresponding to the overlap between the lasing ranges of the two lasers. Again, such synchronization is not commercially available or would be prohibitively expensive.
It has been proposed to combine two cavity lasers with a single, shared tuning device to form a tunable laser apparatus with a tunable range that is approximately equal to the sum of the tuning ranges of the two cavity lasers. Three of the present inventors, and a colleague, disclosed such a widely-tunable laser apparatus in the paper “S-, C- and L-Band Continuously Tunable Fiber Laser Using Thulium- and Erbium-Doped Fibers”, by H. Chen, F. Babin, G. He and G. W. Schinn, Optical Society of America, Conference of Lasers and Electro-optics 2005, Baltimore, May 2005, the contents of which are incorporated herein by reference. The laser illustrated in FIG. 1 of that paper comprises a TDF laser and an EDF laser whose rings are conjoined by two circulators so that a tunable bandpass filter between the two circulators is shared by both rings. Light from the EDF laser propagates through the tunable bandpass filter in one direction and the light from the TDF laser passes through the tunable bandpass filter in the opposite direction.
A somewhat similar approach is disclosed by B. Bouma et al. in United States published patent application No. 2005/0035295 and their paper Wide Tuning Range Wavelength-Swept Laser With Two Semiconductor Optical Amplifiers, IEEE Photonics Technology Letters, Vol. 17 No. 3, March 2005, the contents of both of which are incorporated herein by reference. Their widely-tunable laser apparatus also uses optical circulators to couple a single tunable filter, specifically a diffraction grating and rotatable polygonal mirror, in a common portion of the rings of two cavity lasers.
A disadvantage of both such widely-tunable laser apparatus, however, is that the insertion loss attributable to the circulators limits the useful tuning range of each of the tunable lasers comprising the widely-tunable laser apparatus. This limitation is a consequence of the fact that, in a cavity, the condition that loss is not greater than gain must hold at each wavelength for which lasing action is to occur. Since the gain as a function of wavelength of a given gain medium necessarily falls off as the difference between the desired lasing wavelength and the wavelength corresponding to the gain peak is increased, the ultimate tuning range of the tunable laser may be significantly reduced as the cavity loss is increased. Moreover, it is frequently the case that the insertion loss of a circulator also increases at wavelengths near the extremes of its operating window, thereby further reducing the attainable tuning range of the tunable laser. As a consequence, a widely-tunable laser apparatus based on constituent tunable lasers using circulators in their cavities would need to be comprised of a greater number of tunable lasers to cover a wide spectral range, e.g. 1250-1650 nm, thereby increasing the overall cost and design complexity. There is a need, therefore, for a widely-tunable laser apparatus design, based on tunable lasers, that does not require the use of circulators and minimizes the cavity loss.